Generally described, the efficiency of a gas turbine engine tends to increase with increased combustion temperatures. Higher combustion temperatures, however, may create a variety of problems relating to the integrity, metallurgy, and life expectancy of the components within the hot combustion gas path and elsewhere. These problems are an issue particularly for components such as the rotating buckets and the stationary turbine shrouds positioned in the early stages of the turbine.
High turbine efficiency also requires that the buckets rotate within the turbine casing or shroud with minimal interference so as to prevent unwanted “leakage” of the hot combustion gas over the tips of the buckets. The need to maintain adequate clearance without significant loss of efficiency is made more difficult by the fact that centrifugal forces cause the buckets to expand in an outward direction towards the shroud as the turbine rotates. The bucket tips may erode, however, if the bucket tips rub against the shroud. Such erosion may cause increased clearances therebetween as well as reduced component lifetime. Other causes of leakage include thermal expansion and even aggressive maneuvering of the engine in, for example, military applications and the like.
Abradable coatings have been applied to the surface of the turbine shroud to help establish a minimum or optimum clearance between the shroud and the bucket tips, i.e., the bucket tip gap. Such a material may be readily abraded by the tips of the buckets with little or no damage thereto. As such, bucket tip gap clearances may be reduced with the assurance that the abradable coating will be sacrificed instead of the bucket tip material.
In addition to allowing for the tip-shroud contact, the use of an abradable surface as a pattern of ridges and the like thereon has been found to provide additional aerodynamic benefits in further reducing the leakage flow therethrough. Specifically, the ridges may provide direction to the mainstream flow away from the tip clearance gap. Known abradable patterns thus have been found to provide aerodynamic benefits in the reduction of the minimum tip clearance height and otherwise.
There is thus a desire for an improved abradable bucket shroud pattern so as to reduce the leakage flow through the bucket tip gap and elsewhere. Such an abradable bucket shroud pattern may be optimized for a specific bucket design in terms of the leakage flow therethrough and the heat loads thereon. Specifically, such a bucket shroud design would provide an adequate abradable shroud surface in the context of a flow reducing pattern for improved performance.